javademos/patterns/src/SealedExhaustive.java

public class SealedExhaustive {
    sealed interface Shape permits Circle, Square, Rect {}
    record Circle(double r) implements Shape {}
    record Square(double side) implements Shape {}
    record Rect(double w, double h) implements Shape {}

    // No default: the compiler checks that every permitted subtype is covered.
    static double area(Shape s) {
        return switch (s) {
            case Circle(double r) -> Math.PI * r * r;
            case Square(double side) -> side * side;
            case Rect(double w, double h) -> w * h;
        };
    }

    public static void main(String[] args) {
        for (Shape s : new Shape[]{new Circle(1), new Square(2), new Rect(2, 3)}) {
            System.out.printf("%s -> %.2f%n", s, area(s));
        }
    }
}